1. Field of the Invention
The present invention relates to a vari-focal polar alignment scope applied to an equatorial telescope for an astronomical telescope.
2. Description of the Prior Art
In an equatorial telescope for an astronomical telescope, it is necessary to set the polar axis parallel to the rotational axis of the earth (earth axis). An equatorial telescope having a polar alignment scope wherein the optical axis thereof is aligned with the polar axis in order to facilitate the polar alignment setting is known in the art. The polar axis setting of an equatorial telescope is carried out according to the following method. For example, in the northern hemisphere, the northern sky is observed with a polar alignment scope, and the orientation and high-precision adjustment of the equatorial telescope are carried out in order to set the position of the northern star at a predetermined position from the field-of-view center (optical axis) of the polar alignment scope.
Since the field-of-view is wide when the magnification of a polar alignment scope is low, a target celestial body can be easily guided within the field-of-view, however, the setting precision is low. Conversely, if the magnification is increased to a high level in order to increase the setting precision, the field-of-view becomes narrow, resulting in difficulty in guiding the target celestial body.
Consequently, the inventor of the present invention has proposed, in Japanese Unexamined Patent No. Hei-9-281408, a polar alignment scope which has a converter detachably attached thereto, or a converter which can be inserted and taken out of the optical path of the polar alignment scope, so as to change the magnification. However, it is preferable to provide a polar alignment scope having a wider real field-of-view and a greatly reduced amount of aberrations.